In AC circuits, power factor is defined as the ratio between the real power measured in watts and the apparent power which is the product of the rms voltage and the rms current. The ideal situation is where the power factor is 1, namely that all of the apparent power is real. This occurs when the current drawn from Hydro "looks" like the Hydro voltage, i.e., perfectly sinuosity and in phase with the voltage so that in looking at the two waveforms, it would not be obvious which was which the voltage or the current. With unity power factor the power taken is governed by the formula EQU P=V*I cos (.theta.)*D.F.
where
V=line voltage PA1 I=line current in amperes PA1 .theta.=phase angle between V and I PA1 D.F.=Distortion Factor Total Power/Power of Fundamentals
Since .theta. the phase angle is 0, cos 0.degree. is 1 and D.F. is 1 and P=V*I watts.
If the power factor is less than one, then some or all of the apparent power is not real power and thus is being wasted. In AC circuits the voltage and current are sinusoidal and in phase when the load is a pure resistance or when the impedance of the load is such that the inductance and capacitance of the load balance out and the load is seen as a resistive load. If the impedance of the load is a simple inductive load, the current will still be sinusoidal but will lag the voltage. On the other hand, if the load is a simple capacitive load it will still be sinusoidal but will lead the voltage. In both cases the power factor will be less than unity because Cos .theta. will be less than 1 and would equal 0 for a theoretical circuit of pure inductance or pure capacitance where the phase difference between the voltage and current is equal to 90.degree..
For many years, linear loads were the largest portion of the AC distribution system, the bulk of which were resistive loads from lighting and heating elements and inductive loads from motors. As inductive loads caused the current waveform to lag the voltage waveform, to balance the system it was only necessary to add capacitance to the line. However, this is no longer sufficient.
In recent years, there has been an increase in the number of off-line switching non-linear DC power supply loads on the AC distribution systems of electrical utilities. The proliferation of these non-linear loads has caused problems for the electrical distribution utility systems due to the presence of harmonic distortion as measured by the distortion factor.
The blocking effect of the input diodes and the presence of filter capacitors in such off-line power supplies employing diode rectifiers and filter capacitors causes distortion of the current waveform from a sinusoidal to a complex waveform of tall peaks made up of the fundamental frequency (typically 60 Hz) and a number of harmonics or multiples of the fundamental frequency summed together. As it is only the fundamental waveform that contributes to real power, the presence of harmonics can be detrimental for the utility in many ways. Firstly, utilities only charge for real power used and the harmonics do not contribute to real power. Additionally utilities utilize a common neutral wire in their distribution systems. For linear loads, the return current flows in the neutral wire were algebraically cancelled, resulting in no current flow in the neutral wire. The harmonics generated by non-linear loads cannot be cancelled in the usual manner and in fact are additive which can produce dangerous overloads and heating of the neutral wire and central distribution transformers.
Uncorrected off-line switching power supplies generally have a power factor of about 0.65. This means that total currents of 1.54 (1/0.65) times those of the fundamental current are required to produce the real power and that uncorrected power supplies can only draw 65% of the power of a corrected power supply from the same input current. These high peak currents can trip over-current devices, can cause distortions in the voltage waveforms and can cause clipping of both voltage and current waveforms.
Because of these problems, substantial interest in power factor correction for DC power supplies has emerged. This has led in part to the establishment of standards such as European Community Standard IEC-555-2 IEEE 519, and MIL 1399 which regulate the amount of permissible disturbances caused by appliances and other equipment connected to AC lines.
There have been many proposed methods of power factor correction for DC power supplies including passive filtering, multiphase rectification, rotary reactive correction, shunt electronic correction, series electronic correction and combinations of the above. However, all of these techniques present certain drawbacks. For example, passive filters, which involve filtering of the known frequencies of the harmonics, tend to be large, heavy and expensive and can introduce phase shifts further degrading power factor.
At the present time, most commercial systems for power factor converter use high frequency active waveshaping techniques to achieve performance on a wide range of operating conditions. Many of the schemes use a boosting feature of pulsewidth modulation (PWM) boost class converters, hard switched converters or their resonant-switch counterparts. The series resonance converter has some desirable power factor characteristics. However, due to its voltage step down characteristics, it cannot maintain line current into the valleys of the input AC wave and must be shut off typically when the line voltage falls below 50% of its peak value.
One AC-to-DC converter which draws sinusoidal and in phase current waveforms from the AC source was described in IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL IA-19, pages 586-599, July/August 1983, by Kocher and Steigerwald. The converter was a well-known flyback converter utilizing a field-effect transistor (FET) operating at 45 kHz as a high frequency switch. The shaping of the input current waveform was obtained by modifying the converter to pulsewidth modulate the input power. The control system adjusted the magnitude of the sinusoidal input current to achieve variations in DC output power level. However, the converter described, while achieving a high power factor under high power applications, only exhibited efficiencies of the order of 60 to 70% under low power applications.
A series/parallel LCC resonant circuit in a high voltage boost, wide bandwidth configuration has been described in an overly simplified schematic manner in IEEE 1991, pages 5 to 16 by Schutten et al. The reference describes the simulated performance of a parallel resonant converter and a combination series parallel resonant converter when operated at a fixed frequency achieving a respectable power factor but far from regulatory requirements. However, active control of a series parallel converter is required to maintain zero voltage switching conditions above resonance under all conditions. Switching above and below the resonant frequency based upon the input line current would cause stress on, and probably destruction of, the system component. The reference does not disclose any means for the active control of this system and as far as I am aware no satisfactory control means has ever been proposed.